The Senate Permanent Subcommittee on Investigations is nearly finished with a year-long investigation into the methods that large technology companies use to avoid paying U.S. corporate income tax. Apple, for its part, allocates some 70 percent of its income to overseas affiliates where tax rates are much lower.

It appears that all of Apple’s techniques are legal by U.S. law, though some politicians have said that corporations going to extraordinary lengths to avoid paying income tax and that they are violating the spirit of tax laws.

In its statement, Apple said it paid “an enormous amount of taxes” to local, state and federal governments. “In fiscal 2012 we paid $6 billion in federal corporate income taxes, which is 1 out of every 40 dollars in corporate income taxes collected by the U.S. government,” it said.

Apple was one of the first companies to use the accounting scheme called a “Double Irish With a Dutch Sandwich”, where profits are routed through Irish and Dutch subsidiaries before finally landing in the Caribbean. Now, hundreds of companies use those methods.

Apple also has moved revenue to its Braeburn subsidiary in Nevada and International locales where the company pays little to no tax.

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Apple Inc (AAPL.O) and Twitter are currently not in discussions on the mobile technology giant taking a stake in the popular social networking site, sources familiar with the matter said.

Apple in recent months has held negotiations with Twitter to explore investing hundreds of millions of dollars in the company, The New York Times reported on Friday, citing people briefed on the matter. The Wall Street Journal said such discussion were held over a year ago, citing a person familiar with the matter.

It is unclear if the two companies talked about a deal in the past and at what level such discussions were held, but there are no current, formal talks between the companies on an investment or acquisition, the sources said.

Both Apple and Twitter declined to comment.

The iPhone and iPad maker typically does not take equity stakes in companies and prefers to acquire technologies by buying up smaller startups that are lesser known. Twitter executives have said repeatedly they are in no rush to seek additional financing, either privately or on public markets, since they have “truckloads” of cash.

Apple has never delved deeply into the social media space dominated byFacebook Inc (FB.O), but it has dabbled in trying to make music more social by launching a social network on iTunes called Ping, which has not caught on.

Twitter, the Internet phenomenon with some 140 million users that allows people to “tweet” 140-character messages, is already well integrated into Apple smartphones and tablets. Apple customers can directly share their comments on Twitter when on their iPhones, iPads or Mac line ofcomputers.

Twitter, the San Francisco-based startup that is viewed as the most significant candidate for the public markets following Facebook, is ramping up its efforts to generate revenue from the 400 million tweets that cross its networks daily.

As the cameras and screens of smartphones and tablets improve, and as wireless networks offer higher bandwidth, more companies are getting into the business of enabling mobile video calls.

The details vary from one service to the next, but the experiences are similar: from anywhere in the world with a modern wireless network, a smartphone’s screen fills with the face of a friend or relative. The quality is about the same jerky-but-functional level as most desktop video. Sound is not always perfectly synced with the image, but it is very close. The calls start and end the same way, by pressing a button on the screen.

Mobile video calling has risen so quickly that industry analysts have not yet compiled exact numbers. But along the way, it is creating new business models, new stresses on mobile networks and even new rules of etiquette.

Once an interesting endeavor for a few start-ups like Tango, mobile video has caught the attention of big companies. Apple created FaceTime and made it a selling point for the iPad. In September, the company made FaceTime available on cellular networks instead of limiting it to Wi-Fi systems, almost certainly in response to increasing consumer demand.

Last week, Yahoo purchased a video chat company called OnTheAir. And in 2011,Microsoftpaid $8.5 billion for Skype, a service for both video and audio-only calls. Though most people use Skype on desktop and laptop computers, the software for the service has been downloaded more than 100 million times just by owners of phones running Google’s Android mobile operating system. Microsoft built a service for its Windows 8 mobile phone that lets people receive calls even when Skype isn’t on.

Google, which has more than 100 million people a month using its Google Plus social networking service, now offers more than 200 apps for its video calling feature. It says it is interested not in making money on the applications, but in learning more about them so it can sell more ads by getting people to use its free video service, called Hangouts. Hangouts can be used for two-person or group calls, or for a video conference with up to 10 people.

“On a high level, Google works better when we know who you are and what your interests are,” said Nikhyl Singhal, director of product management for Google’s real-time communications group. “Video calling is becoming a basic service across different fronts.” While Mr. Singhal is an occasional user, he said, his 4-year-old daughter “is on it every day.”

Don’t expect video calling to improve productivity. Tango uses the same technology that enables video calls to sell games that people can play simultaneously. It sells virtual decorations like balloons to drop around someone’s image during a birthday call (both parties see the festive pixels). Google says some jokey applications on Hangouts, like a feature that can put a mustache over each caller, seem to encourage people to talk longer.

Currently, popular two-way games like Words With Friends on Facebook work by one player making a move and then passing the game over to the other player, not watching moves as they are made. Another promising area is avatars, like cartoon dogs and cats, that mouth speech when a user wants to have a video call but doesn’t want to be seen.

The prospect of having to appear on-screen at any given moment might sound like a nonstarter for people who worry about bad hair days. But in fact, using mobile devices for video calls may be less bother than it seems. “There may be natural inhibitions to being seen, but when I’m on a mobile device I’m out and about, so I’m more likely to be presentable,” said Michael Gartenberg, a consumer technology analyst at Gartner. “How people use this remains to be seen, but they are starting to expect it.”

Yet a new etiquette for mobile video calls is already emerging. People often text each other first to see if it’s O.K. to appear on camera. Video messages sent in the text box of a phone, like snippets of a party or a child’s first steps, are also useful precursors to video conversations. Mr. Singhal said making avatars for users of Hangout would be “an extraordinarily important area” as well.

The greatest challenge for the business may not be getting more consumers to use the service, but making sure the service works. Most phones have slight variations in things like camera placement and video formatting from one model to the next. “A camera can show you upside down if you load the wrong software on it,” said Mr. Setton of Tango.

As a result, the 80 engineers among Tango’s 110 employees have adjusted their software to work on more than 1,000 types of phones worldwide. The top 20 models have more than a million customers each, but the complexity of building software for a wider range of phones has made it hard for new mobile video companies to enter the field, Mr. Setton said.

Tango’s average video call used to last six minutes, Mr. Setton said, but when the company started adding other applications to go with the videos, like games and designs that float over people, the average call length rose to 12 minutes.